Field of the Invention
The present invention relates to an image display apparatus and a control method thereof.
Description of the Related Art
Lately the image quality of liquid crystal displays is becoming progressively advanced, and the level of user' demands for stability in display devices and gradation of display images (images displayed on a screen (display surface)) is escalating daily.
However the display characteristics of liquid crystal displays change due to age related deterioration, and this change of display characteristics changes the gradation of display images. Therefore in order to display images that always have stable gradation, it is necessary to periodically calibrate the display characteristics. Particularly in the case of medical display devices used for diagnosis, such a change in the gradation of images could affect diagnosis, so insuring the stable gradation of images is a critical issue.
One calibration method is using an optical sensor that detects light from a part of a region of the screen (Japanese Patent Application Laid-Open No. 2007-34209). In concrete terms, calibration is performed using detected values by the optical sensor acquired when an image for calibration is displayed on this part of the region. According to the technique disclosed in Japanese Patent Application Laid-Open No. 2007-34209, the optical sensor can be housed in a bezel part, and placed in a position facing the screen only when calibration is executed. Therefore the optical sensor never obstructs a part of the screen except when executing calibration. In other words, the optical sensor never interrupts the visibility of a displayed image.
Light emitting diodes (LEDs), which have a long life span and low power consumption, lately are used as the light source of the backlight of liquid crystal displays.
Further, a known control method entails a backlight constituted by a plurality of light emitting units each of which has one or more LEDs, and increasing the contrast of the display images by individually controlling the light emission quantity (light emission intensity) of the plurality of light emitting units in accordance with the brightness information (e.g. statistical amount of brightness) of the input image data. This control is normally called “local dimming control”. In local dimming control, the light emission quantity of the light emitting units corresponding to a bright region is set to a high value, and the light emission quantity of the light emitting units corresponding to a dark region is set to a low value, whereby the contrast of the display image is enhanced.
However if the calibration is executed during local dimming control, the light from the part of the region (region that emits light to be detected by the optical sensor) changes due to the difference of the light emission quantity between the light emitting units, and error in the detected value by the optical sensor sometimes increases. This may result in the inability to perform accurate calibration. Details on this will now be described.
It is known that in local dimming control, the contrast of display images can be enhanced, but this causes a halo phenomenon to occur.
FIG. 14 shows an example of an input image 1401, a display image 1402 and a backlight emission pattern 1403.
If the input image 1401 (image of a white object on black background) is inputted, the light emission quantity of light emitting units (LED_Bk) corresponding to the region where the black background is displayed, out of the screen region, is set to a low value due to local dimming control. Then the light emission quantity of light emitting units (LED_W) corresponding to the region where a white object is displayed is set to a high value. Thereby the contrast of the display image can be enhanced.
However, because the difference of the light emission quantity between LED_Bk and LED_W is large, light from LED_W is leaked into a region corresponding to LED_Bk, and a halo phenomenon is generated in the region A of the display image. The halo phenomenon is a phenomenon where a dark region around a bright region is brightly displayed, and in the case of FIG. 14, due to the halo phenomenon, a black brightness in the black background is displayed at a higher level. In other words, the light from the region A is changed by the light from the light emitting units corresponding to the peripheral region.
In the case of FIG. 14, the light from a region including a part of the region A where the halo phenomenon is generated is detected by the optical sensor, which increases error in the detected value determined by the optical sensor, and makes it difficult to perform accurate calibration.